1. Field of the Invention
This invention relates generally to a two-needle overedge sewing machine used for making stitches enwrapping the edge of a fabric material, and particularly to apparatus for backtacking a thread chain formed and cut after the last overedging work so that the thread chain is sewn into overedge stitches applied to a subsequent fabric material.
2. Description of Prior Art
Overedge seams formed by a two-needle overedger or an overedge (overlock) sewing machine used for enwrapping the edge of a fabric material, comprise needle threads, an upper looper thread and a lower looper thread. When a trailing edge of the fabric material is reached, then overedge sewing is terminated and the threads are woven to form a bar-shaped thread chain which extends over a desired length determined by the operating time and speed of the sewing machine. The fabric material is then turned around together with the thread chain extending from the trailing edge of the fabric material, to be brought to the operator's side with respect to the needles. The thread chain is stretched and is held by a clamp having a cutter to cut the thread chain. A portion of the thread chain extending from the clamp toward the needles is held on a throat plate, so that this portion of the thread chain will be sewn into overedge stitches applied to a subsequent fabric material subjected to overedging.
Generally speaking, such an overedger comprises two loopers as means for providing threads for forming enwrapping seams on a fabric material. During overedging, one looper, i.e. lower looper, moves from the position of a needle toward the edge of the fabric material in a direction normal to the line of seams below the material, while the other looper, i.e. upper looper, moves above the material from the edge thereof to intersect the passage of a needle. Such movement of the two loopers act to form overedge stitches, to enwrap the edge of the fabric material.
A two-needle overedger comprises two sewing needles arranged in parallel to each other. The throat plate used in such an overedger has an inner finger positioned between inner and outer needle descending points and an outer finger positioned beside the inner finger. The outer needle descending point is interposed between the inner and outer fingers which extend parallel to each other. One example of such a conventional two-needle overedge sewing machine is disclosed in U.S. Pat. No. 4,356,782.
When a thread chain is formed immediately after overedging work and the thread chain is pulled straight with the fabric turned around to be positioned at the operator's side to cut and clamp the thread chain, the remaining thread chain extending on the surface of the throat plate from the clamp-cutter is located close to the outer needle descending point. Therefore, when subsequent overedging work is started on another fabric material, the thread chain is pulled toward the needle as the needle descends. As a result, the thread chain is apt to be positioned at a position where the outer sewing needle descends. In such a case, the outer sewing needle penetrates the thread chain. Once the thread chain is sewn, enwrapping operation of the thread chain is interrupted and the greater part of the thread chain protrudes beyond the leading edge of the fabric material. In other words, the greater part of the thread chain remains unsewn without being backtacked. Furthermore, when the thread chain is in the above-mentioned state, the formation of loops by means of the upper looper is obstructed, since the loops are arranged to be formed on the upper surface of the fabric such that the upper looper moves so that the looper thread engages the needle thread.
As a result, some stitches formed by the overedge are skipped.